A sound-damping treatment for noise component included in an exhaust gas is carried out by disposing a muffler in a halfway of an exhaust tube.
As the muffler, there are known various structures, among which an adequate structure is used in accordance with conditions such as a displacement of an engine and the like. As the muffler for damping noise component of high frequency among various noise components included in the exhaust gas, there is well-known a structure that an inorganic fiber sound absorbing material is disposed around a metal tube (inner tube) provided with a plurality of small holes and then covered with a metal shell.
In such a muffler, glass fibers having a low heat resistance and the like are usually used as the inorganic fiber sound absorbing material. Recently, the rise of exhaust gas temperature becomes conspicuous with the advancement of engine performances, and hence the glass fibers are fused and shrunk by heat of the exhaust gas to form beads. On the other hand, pressure shock accompanied with the passage of the high-temperature exhaust gas concentrates in the small holes of the metal tube and hence the metal tube vibrates or the passing exhaust gas pulsates. Thus, the bead-shaped glass fiber is put through the small holes into the inside of the metal tube and scattered to the outside together with the exhaust gas. Therefore, this muffler has a problem that the sound damping effect is considerably degraded.
In order to solve the above problem, a muffler 1 as shown in FIG. 1 has been proposed in JP-U-61-59819 and JP-Y-6-19785. In the muffler 1, a metal cushioning material 6 such as stainless wool is interposed between a metal tube 3 provided with a plurality of small holes 2 and a sound absorbing material 5 covered with a metal shell 4 and made from glass fibers.
Since the muffler 1 is to damp noise components, however, the metal cushioning material 6 is rendered into an interconnecting cell structure and does not thermally protect the glass fiber sound absorbing material 5. Further, the metal cushioning material is softened by heat of the exhaust gas, so that the effect as a cushioning material is very low. As a result, the conventional muffler 1 has a drawback that it is difficult to damp noise components over a long period.
And also, there is disclosed a method of improving the heat resistance by using silica-alumina ceramic fiber or general-purpose crystalline alumina fiber as the inorganic fiber sound absorbing material. However, the silica-alumina ceramic fiber contains about 50 wt % of granulated substance called as shot, so that there is caused a problem that the shot is moved inside the sound absorbing material by vibration to form spaces in the sound absorbing material. On the other hand, the general-purpose crystalline alumina fiber is a refractory heat-insulating material usually used as a thermal insulant for a high-temperature ceramic furnace of about 1400.degree. C. and has an average fiber size of 2.7-3.2 .mu.m, which is finer than an average fiber size of the conventional glass fiber of about 9 .mu.m, and a high true specific gravity. For this end, the pressure drop becomes higher and particularly there is caused a problem that sound absorption coefficient at a high frequency side is considerably low.
On the contrary, there is a method of increasing a filling density of the general-purpose crystalline alumina fiber. However, as the filling density increases, mass as a sound absorbing layer becomes higher and noise hardly enters into the sound absorbing layer and hence there is caused a problem that the sound absorption coefficient as a whole lowers.
In general, the muffler has a structure that both ends of the metal shell having a diameter larger than that of the metal tube are size-reduced to approximately an outer diameter of the metal tube and fixed to an outer periphery of the metal tube at each opening portion of the metal shell through welding and the sound absorbing material is filled in a space defined between the metal tube and the metal shell. In such a structure, the metal tube is compressed to cause buckling due to the difference in thermal expansion between the metal tube exposed to the high-temperature exhaust gas and the metal shell exposed to air, or the weld portion between the metal tube and the metal shell is peeled off to cause the leakage of the exhaust gas and hence there is a problem that sound radiating the exhaust gas becomes large.
In order to solve the above problems, there is known a structure that the welding between the metal tube and the metal shell is carried out at either an opening portion at both ends of the metal shell, while a mesh-shaped stainless gasket is previously attached to the other opening portion of the metal shell and interposed between the metal tube and the metal shell, whereby the influence of the thermal expansion difference between the metal tube and the metal shell is eliminated to prevent the leakage of the exhaust gas to thereby control volume of radiating sound. However, the gasket should be previously fixed to the metal shell by spot welding or the like, so that the number of assembling steps increases and also the cost increases.